Large sections of the design (specifically the arrow) have no features, instead relying entirely on textures to give the model depth and weight. That'd certainly speed up rendering time, but the lack of shadows and detail/angles have a tendency to make things look toylike on a lower budget, especially on an HD screen. That may be why they decided to have a cutout section in the saucer. It's where the camera will be most focused on and as such needs some depth to it. That or the overall model just looked boring. The nacelles are overly elongated (and again lacking in detail), but I guess they were trying to focus on making it look good in limited angles. The Shenzou is strangely the opposite for a non-hero ship. It's model is highly detailed, with different angles, features and no large flat spaces. It actually looks like it's had the most effort put into it. For the Discovery possibly. Just as likely is that they're still rendering them or they don't like the scenes they've rendered. For the Shenzou a lot of her scenes are probably spoilery, so yeah, they'll be saving them.